1. Field of the Invention
The present invention relates to vehicle navigation, and specifically to route guidance for vehicle navigation to one of a category of places.
2. Description of the Related Art
In general, a navigation apparatus installed in a motor vehicle has a map data storage device such as a compact disk read-only memory (CD-ROM) or an integrated circuit (IC) memory card in which map data is stored, a display unit, and vehicle movement detection devices, such as a gyroscope, a receiver in a global positioning system (GPS) and a vehicle speed sensor, for detecting a current position and a current heading of the vehicle. Map data containing the current position of the vehicle is read out from the map data storage device, a map image representing an area around the actual vehicle position is drawn on the display unit on the basis of the map data, and a vehicle position mark (indicating the location of the vehicle) is also displayed on the display unit by being superposed on the map image. As the vehicle travels, the map image scrolls or the vehicle position mark moves while the map image is fixed on the display, so the driver can recognize where he is currently travelling at a glance.
Ordinarily, this kind of vehicle navigation apparatus includes a route guidance function, enabling the driver to easily drive to a desired destination without deviating from a guide route. This route guidance function uses a process of automatically searching for a shortest route from a starting point to a destination by simulation calculation using the breadth-first search method, the Dijkstra method or other methods using map data, storing guide route data representing the shortest route thereby searched out, displaying on a display screen a corresponding map image in the form of a thick line or in a color different from the color in which other roads are displayed, and, when the vehicle is within a certain distance from an intersection where the heading of the vehicle is to be changed, displaying an arrow indicating the desired course together with a map image representing the intersection, thereby enabling the driver to easily understand the optimal route to the destination.
Map data stored in the map data storage device, e.g. a CD-ROM, is sectioned by suitable longitude and latitude widths at reduced scales, for example, 1/12500, 1/25000, 1/50000 and 1/100000. In the map data, data pertaining to roads is stored as sets of coordinates representing vertices (nodes) in terms of longitude and latitude. Each road is defined by two or more nodes connected to each other; a connection of two nodes is called a "link". The map data is (1) a road layer for map matching and route search, including a road list, a node table, an intersection forming node list and an intersection network list, (2) a background layer for displaying roads, buildings, facilities, parks, rivers and so on in map pictures, and (3) a letter/symbol layer for displaying letters representing names of entities such as cities, towns and villages, roads, intersections, and buildings as well as map symbols.
Further, a CD-ROM for map storage has recently become available in which facilities and stores, such as parking areas, convenience stores and public restrooms, are each recorded as an element of a category of similar places. When a user selects a destination in a category of places (e.g., convenience stores) in the navigation apparatus using such a CD-ROM, the navigation apparatus can search out a convenience store located near the user's present vehicle position, so as to display a corresponding shortest route to the convenience store on the display unit.
However, even if the nearest convenience store is reached via a suitable guide route, there is a possibility of e.g. the store being closed. Then the user must search for another similar destination by resetting the category of places. There is also a difficulty in performing suitable navigation for example, if traffic congestion occurs on the route, or passage on the guide route is inhibited by traffic control. Further, a driver may be unable to park his vehicle after arrival at a parking area searched out by the navigation apparatus, if no space is available in the parking area.
On the other hand, an advanced traffic information service (ATIS) system using cellular telephony and a vehicle information and communication system (VICS) using a radio signal beacon, a light beacon or frequency modulation multiplex broadcasting, have been developed as systems that externally transmit traffic information directly to vehicle navigation apparatuses. The ATIS system is already in practical use in certain countries. Information provided by the ATIS includes (1) traffic congestion information, (2) accident information, (3) road construction information, (4) road closing information, (5) ramp/interchange closing information, (6) parking area full/empty information, (7) traffic regulation enforcement information, (8) traffic control information, and (9) administration information. With respect to the VICS, substantially the same information service is planned.
A navigation apparatus using the ATIS displays a congested road, for example, in red in the map image. A driver using such a navigation apparatus can avoid involvement in traffic congestion by driving away from a congested road displayed on the display unit. However, even when a destination is reached by suitable route guidance, if the driver wishes to go to another destination, e.g. another convenience store, it is necessary for the driver to again perform the destination category setting process.